Comprehensive guide to diagnosing and resolving common issues with Azure Virtual Networks.
Common Virtual Network Issues and Solutions
This document provides a structured approach to identifying and resolving common problems encountered when working with Azure Virtual Networks (VNet). We'll cover connectivity issues, performance bottlenecks, security configurations, and more.
1. Connectivity Problems
Connectivity issues are the most frequent challenges. This section outlines how to diagnose and fix them.
1.1. Cannot Reach Resources Within the Same VNet
If you cannot connect to a virtual machine or other resource within the same VNet, check the following:
Network Security Groups (NSGs): Ensure NSG rules allow traffic between the source and destination IP addresses and ports.
Firewall Rules: Verify that the operating system's firewall on the VM is not blocking traffic.
Subnet Configuration: Confirm that both the source and destination resources are in the correct subnets and that there are no route table issues.
1.2. Cannot Reach Resources in Another VNet (VNet Peering)
When VNet peering is configured, issues often stem from the peering configuration itself or associated NSGs/firewalls.
Peering Status: Check that the VNet peering connection is 'Connected' in the Azure portal.
Gateway Transit: If one VNet uses a virtual network gateway for on-premises connectivity, ensure 'Allow gateway transit' is enabled on the peered VNet and 'Use remote gateways' is enabled on the VNet that has the gateway.
Address Space Overlap: Ensure there is no overlap in the address spaces of the peered VNets.
Troubleshooting VPN gateway connectivity requires checking both Azure and your on-premises network.
Gateway Status: Verify the VPN gateway status in the Azure portal is 'Succeeded'.
Connection Status: Ensure the connection status is 'Connected'.
On-Premises Firewall: Confirm that your on-premises firewall allows traffic on the necessary VPN ports (e.g., UDP 500, 4500 for IKE).
Public IP Addresses: Ensure the Azure VPN gateway's public IP and your on-premises device's public IP are correctly configured in the connection settings.
IPsec/IKE Policies: Mismatched encryption or hashing algorithms can prevent connections. Ensure they are compatible.
Note: Use the Azure Network Watcher's IP Flow Verify and Connection Troubleshoot tools for detailed insights into connectivity.
2. Performance Issues
Slow network performance can impact application responsiveness. Identify potential bottlenecks.
2.1. Low Throughput
Factors affecting throughput include VM size, NSG processing, and network path congestion.
VM Network Bandwidth: Ensure your VM's size supports the required network throughput. Larger VMs generally have higher network limits.
NSG Performance: While Azure NSGs are efficient, a very large number of complex rules could potentially introduce minor latency.
Network Path: For inter-VNet or hybrid connections, the longest network path or intermediate hops can affect speed.
2.2. High Latency
Latency is often related to physical distance and network hops.
Region Selection: Deploy resources in the Azure region closest to your users or on-premises location.
Service Endpoints/Private Link: For accessing Azure services, consider using Service Endpoints or Private Link to keep traffic within the Azure backbone, reducing latency.
Tip: Use Azure Network Watcher's Packet Capture and Performance Monitor to analyze network traffic patterns and identify latency sources.
3. DNS Resolution Problems
Incorrect DNS resolution can prevent resources from being found by name.
Azure-Provided DNS: Ensure your VNet is configured to use Azure's default DNS servers (168.63.129.16) if you're not using custom DNS servers.
Custom DNS Servers: If using custom DNS servers (e.g., on-premises domain controllers), verify their reachability from the VNet and that they are correctly configured to resolve Azure internal/external hostnames.
Conditional Forwarders: If your custom DNS servers are authoritative for some domains but need to forward requests for others (like Azure resources) to Azure DNS, ensure conditional forwarders are set up correctly.
4. Security Configuration Errors
Misconfigured security can block legitimate traffic or allow unauthorized access.
4.1. Inbound/Outbound Connectivity Blocked
This is typically due to NSGs or Azure Firewall rules.
NSG Rule Review: Carefully examine inbound and outbound NSG rules applied to the VM's network interface or subnet. Use the NSG Flow Logs to see which rules are being hit.
Azure Firewall Rules: If using Azure Firewall, check its network and application rules for any deny statements that might be blocking traffic.
4.2. Access to Azure Services Blocked
Ensure you can reach necessary Azure service endpoints.
Service Endpoints: If Service Endpoints are used, confirm they are enabled on the subnet and that the NSG/Firewall rules allow access to the specific Azure service.
Private Endpoints: If Private Endpoints are used, verify DNS resolution is correctly pointing to the private IP address and that NSGs/Firewalls allow traffic to the Private Endpoint's subnet.
Important: Always test connectivity from the source VM using tools like ping, telnet, or Test-NetConnection (PowerShell) after making NSG or Firewall changes.
5. Advanced Troubleshooting Tools
Azure provides powerful tools to help diagnose complex network issues.
Network Watcher:
IP Flow Verify: Determines if traffic is allowed or denied to or from a VM.
Connection Troubleshoot: Tests connectivity between two endpoints.
Packet Capture: Captures network packets to and from a VM.
Topology: Visualizes the network topology, including VNets, VMs, gateways, and peering.
NSG Flow Logs: Provides insights into traffic flows through NSGs.
Azure CLI / PowerShell: Use commands like az network vnet list, az network vpn-connection list, Get-AzVirtualNetwork, and Test-AzNetworkConnectionPacket for programmatic diagnostics.
By systematically applying the steps and tools outlined in this guide, you can effectively troubleshoot and resolve most virtual network-related problems in Azure.